ABSTRACT
As a defect or impurity in a semiconductor, hydrogen would appear to be conceptually simple. Is it not simply a question of the material’s response to a unit test charge - the interstitial proton? In fact, its behaviour is notoriously complex. Most widely known is its propensity to pair with and passivate other impurities and defects [1]. It does this so readily that isolated hydrogen defect centres are exceedingly difficult to study spectroscopically. Nonetheless it is now realised that isolated H+, H° and H“ centres can exist, albeit transiently, and that the interplay between them as charge carriers are captured or lost gives hydrogen a significant electrical activity of its own. Only in silicon is this amphoteric behaviour reasonably well documented. Donor and acceptor functions are known, with corresponding energy levels lying deep within the energy gap [2]. Evidently hydrogen does not then constitute a ‘hydrogenic’ shallow donor! That is, in silicon at least, it does not show a weakly bound state which can be described within the effective mass model, in the manner of interstitial lithium or substitutional phosphorus. The proton charge is not effectively screened by the bulk dielectric constant.
